code: def helper_test_freq_coversion(self, test_cases):
      for freq, period in test_cases.items():
          ts_sine = tg.sine_timeseries(
              value_frequency=1 / period, length=3, freq=freq
          )
          # this should not raise an error if frequency is known
          _ = Prophet._freq_to_days(freq=ts_sine.freq_str)
  
      assert (
          abs(
              Prophet._freq_to_days(freq="30S")
              - 30 * Prophet._freq_to_days(freq="S")
          )
          < 10e-9
      )
  
      # check bad frequency string
      with pytest.raises(ValueError):
          _ = Prophet._freq_to_days(freq="30SS")
  
file_path: darts/darts/tests/models/forecasting/test_prophet.py
imports: from unittest.mock import Mock
  import numpy
  import pandas
  import pytest
  from darts import TimeSeries
  from darts.logging import get_logger
  from darts.models import NotImportedModule, Prophet
  from darts.utils import timeseries_generation
label_desc: Use image node_id 7 for calling the TestProphet obj's underlying member method code with example usage: obj.helper_test_freq_coversion(test_cases) without return types